1. Field of the Invention
This invention relates to an ionic electrolyte membrane structure, a method for its production and a solid oxide fuel cell making use of the ionic electrolyte membrane structure.
2. Description of the Related Art
As our living standard becomes higher, they use electrical fittings in more various ways of living and come to prefer brighter rooms than ever, so that, correspondingly thereto, the consumption of electricity for lighting has been increasing. The consumption of electricity has also been increasing with a rise in the Internet connections and digital communications, making use of personal computers. However, in the present state of affairs, the construction of power plants or stations does not always turn out as desired, and the situation is that, in order to meet such increasing demand for electricity, the utilization of reproducible energy such as solar-cell electricity has to be taken into consideration.
It is difficult in the present state of affairs to construct large-scale power plants and also the loss in electricity during its transmission cannot be ignored in conventional power generation and transmission systems. Accordingly, distributed (or on-site) power generation is considered to be of a great trend hereafter. A system in which solar cells are installed in homes to cover much of the home power consumption is considered to come to be an important way. The utilization of fuel cells making use of town gas or the like is also considered to come to be an important choice.
Now, there are various types in the fuel cells. In a polymer electrolyte fuel cell (hereinafter often also “PEFC”), which is a prevalent fuel cell at present, it necessitates water as an ancillary mechanism for the conduction of protons (hydrogen ions), and hence it is restricted to its use at 100° C. or below only. Its actual use is said to be at 80° C. or below, and the fuel cell has been found to have a limitation on how the heat be utilized and have a problem on its lifetime because a thin polymer layer is used as an electrolyte membrane.
Meanwhile, as a type considered optimal for users of medium-scale electricity as in homes, convenience stores and other various shops, a solid oxide fuel cell (hereinafter often also “SOFC”) is available. The SOFC is basically made up of a solid electrolyte capable of transmitting ions selectively and, provided respectively on both sides thereof, two electrodes (an air pole and a fuel pole) holding the former between them. Then, oxygen is flowed through the air pole and hydrogen through the fuel pole whereby chemical reaction proceeds and electricity is generated. The electrolyte may at least be a material capable of transmitting either of oxygen ions and hydrogen ions. Usually, in view of restrictions on materials, the material capable of transmitting oxygen ions is used. Stabilized zirconia in which yttrium oxide is added to zirconium oxide so as to make its structure stable (hereinafter often also “YSZ”) is used as the electrolyte material. Lanthanum manganite having perovskite structure and part of lanthanum of which has been substituted with an alkaline earth metal, [La1—X(M)X]YMnO3 (M: an alkaline earth metal), is used for the air pole, and as the fuel pole, nickel zirconia cermet is used which is prepared by mixing YSZ with a stated amount of Ni.
As structure of the SOFC, what is known is a structure in which, as shown in FIG. 1, a single cell 40 made up of a solid electrolyte 41 and provided respectively on both sides thereof an air pole 42 and a fuel pole 43 is set in layers on an interconnector 44 having fuel passages 47 and oxidizing agent passages 48 and the electrolyte of which is provided with mechanical strength.
Such an SOFC is advantageous in view of lifetime because it makes use of an inorganic material for the electrolyte, and can utilize heat in virtue of its high service temperature, where its total efficiency is 50% or more, which exceeds the efficiency of about 35% the PEFC has. In addition, although the PEFC requires use of platinum as a catalyst, which is expensive, the SOFC, which is used at a high temperature of 200° C. or above, does not at all require use of any catalyst such as platinum, or makes it enough to use it in a smaller amount than any PEFC at least which is driven at room temperature. This is characteristic of the SOFC, thus the SOFC can be said to be superior in this respect as well, and is sought to be put into practical use.
In the conventional SOFC, as stated above the YSZ or the like is used as the electrolyte material and the conduction of oxygen ions is utilized, where, because of a low ionic conductivity, the fuel cell requires a certain degree of high temperature in order to secure any necessary ionic conductivity, and usually the electricity is generated at about 800° C. However, such a high temperature as service temperature may cause a great temperature difference in the interior of the fuel cell, and it has sometimes come about that this temperature difference causes the fuel cell to break due to a difference in thermal expansion.
As a countermeasure therefor, the SOFC must be made to slowly change in temperature over a period of many hours when it is started or stopped. Accordingly, the SOFC is considered not usable in homes and the like where its on-off is frequently repeated, and is considered suitable for its use in convenience stores and the like where the electricity is continuously used day and night. In addition, because of its use at a high temperature, it may break, besides its break due to the difference in thermal expansion, due to growth in size or change in shape of particles in the interior of its electrolyte membrane. Thus, in order to make an SOFC with a high energy efficiency usable in a broader range inclusive of its use in general homes, it is deemed to be a subject, for the above reasons, how the SOFC be made usable at a lower temperature. Hence, it is sought to develop a material having a high ionic conductivity even at 500° C. or below, or at much lower temperature, i.e., a temperature close to room temperature.
Now, as prior art, Non-patent Document 1 (J. Garcia-Barriocanal, A. Rivera-Calzada, M. Varela, Z. Sefrioui, E. Iborra, C. Leon, S. J. Pennycook, J. Santamarial; Science, 321 (2008) 676: “Colossal Ionic Conductivity At Interfaces Of Epitaxial ZrO2: Y2O3/SrTiO3 Heterostructures”) discloses that, where the YSZ (8 mol % of Y2O3 is mixed) that has conventionally been available as an oxygen ion conductive material and an ion non-conductive material SrTiO3 (hereinafter often simply “STO”) are mutually layered, oxygen is conducted through the interface between the two layers and the fuel cell shows an ionic conductivity of as high as 1×102 S/cm even at about 200° C.
Usually, fuel cells are considered necessary for them to have an ionic conductivity of 1×10−2 S/cm or more in a service temperature range, and hence, the above ionic conductivity of 1×102 S/cm can be said to be a sufficient ionic conductivity. Also, in the double-layer membrane proposed as above, it follows that, as long as ions are to be conducted by equal distance, the amount of ionic conduction more increases as the number of interfaces is made larger as far as possible. Then, in the above way of conduction, where such membranes are formed in laminae, the oxygen is conducted through the interior of interfaces between multiple layers, and therefore the ions are conducted in parallel to the membranes formed in laminae. However, in usual fuel cell electrolyte membranes or ion separation membranes, ions must be conducted in the direction perpendicular to the membranes, thus the double-layer membrane proposed as above is required to be structurally remedied in order for it to be put into practical use.